The present invention relates to a master making device for a stencil printer and, more particularly, to a master making device capable of transporting a web or stencil toward a master clamper openably provided on a print drum in a stencil printer.
Generally, a master making device incorporated in a stencil printer is made up of a thermal head for cutting a web or stencil to form a document image, a platen roller for pressing the web against the head, a first roller pair for transporting the web toward a printer body, a second roller pair for transporting the leading edge of the web being driven by the first roller pair toward a master clamper openably provided on a print drum included in the printer body, and a cutter for cutting off the web in a predetermined size. The print drum accommodates therein an ink supply tube for supplying ink, an ink roller to which the ink is applied, and a doctor roller for leveling the ink on the ink roller. Disposed in the printer body are a pair of register rollers for feeding a recording medium in the form of a sheet to the print drum at predetermined timing, a press roller for pressing the sheet against the stencil or master wrapped around the drum, and a separating member for separating the sheet or printing from the drum.
Japanese Utility Model Publication No. 33343/1989 discloses a rotary stencil printer having a master making device of the type causing a web to form a slack between two consecutive roller pairs (corresponding to the above-mentioned roller pairs). The web is wrapped around a print drum with the first roller pair being rotated. Japanese Patent Laid-Open Publication No. 188265/1985 proposes a tension adjusting method which causes a web to form a slack between a restricting member of a take-up device and a transport member (corresponding to the two roller pairs). This method adjusts the take-up speed of a print drum and the transport speed of the transport member such that the slack of the web does not exceed a predetermined size. Japanese Patent Laid-Open Publication No. 248282/1990 teaches a thermosensitive stencil transporting method which prevents electrostatic charges from depositing on transport rollers (corresponding to the two roller pairs) or on a stencil by reducing the contact area of the transport rollers and stencil or by use of a conductive brush or roller. Further, Japanese Utility Mode Laid-Open Publication No. 82566/1990 describes a stencil printer having a guide member for opening and closing a stock portion which receives a slack of a stencil and intervenes between a feed roller pair and a discharge roller pair. When the guide member opens the stock portion, a tension member applies a predetermined tension to the stencil.
In any one of the conventional master making schemes described above, assume that the web is fed at a speed higher than the rotation speed of the print drum while the former is wrapped around the latter. Then, the web wrapped around the drum will slacken and crease. This problem can be eliminated only if the linear velocity of the web and that of the drum are accurately synchronized. However, since the rotation of the drum is apt to become irregular, it is, in practice, difficult to synchronize the linear velocities of the web and drum accurately. In the light of this, it is a common practice to stop the rotation of the second roller pair at the time when the web is wrapped around the drum. In this condition, the second roller pair follow the rotation of the drum due to the pulling force of the web. As a result, an adequate degree of tension is applied to the web to prevent it from slackening on the drum.
However, causing the second roller pair to follow the rotation of the drum as stated above is undesirable since the rotation of the drum is irregular. Specifically, the irregular rotation of the drum changes the transport speed of the web relative the head, effecting the quality of the resulting master. Moreover, the irregular rotation is apt to exert an excessive load or tension on the web and break it. To eliminate these problems, the first roller pair is located between the second roller pair and the platen roller and rotated at a higher peripheral speed than the second roller pair. In this configuration, the web is caused to slacken between the two roller pairs, so that the excessive tension of the web due to the irregular rotation of the drum may be absorbed.
As stated above, it has been customary with a master making device to assign a plurality of different functions to the second roller pair, i.e., the function of feeding the leading edge of the web to the master clamper of the drum, the function of causing the web to slacken, and the function of applying a tension to the web when the web is to be wrapped around the drum. As a result, the arrangement for transporting the web is complicated and difficult to adjust. This increases the overall cost of the printer and obstructs efficient maintenance, i.e., degrades reliability against errors.
On the other hand, the platen roller has the function of transporting the web at a predetermined speed in addition to the function of pressing it against the surface of the head. It follows that the platen roller originally can transport the web alone and will implement an extremely simple master making device if the other transport members are not used. However, when the platen roller is used alone, it slackens and creases the web and effects the quality of the resulting master, as stated earlier.
By a series of extended researches and experiments, we found a method which is a solution to all of the problems discussed above.
However, although the transport system using only the platen roller to drive the web as described above successfully eliminates the various problems, it brings about another problem, as follows. The position where a transporting force acts on the web is remote from the position where the master clamper clamps leading edge of the web to the upstream side. Hence, on a transport path defined by a pair of guide plates, the transport of the leading edge of the web is extremely unstable. As a result, the leading edge of the web often jams the transport path between the guide plates and cannot be surely transported to the master clamper.